Due to their usage or their environmental exposure biodegradable materials often contain a sufficiently versatile biocenosis of microorganisms which biologically degrade these materials completely or partly, depending on the environment concerned, meaning that biodegradation spontaneously materializes in digestion of these materials. This applies especially for wastes having a relevant content of the biogenic-organic materials containing, simply from their origin, a corresponding concentration of microorganisms.
Where an adequate supply of oxygen exists, the carbon bound in the organic substance is converted into cell mass and carbon dioxide, these products corresponding to the natural products of metabolism in the earth's atmosphere. If, however, the oxygen supply is insufficient to ensure aerobic conditions, anaerobic biodegradable processes occur which ultimately can result in methane being generated. When this happens, methane is emitted in open storage of the matter, a gas which in the ambient air poses a relevant danger of explosion and has a high greenhouse potential. This is why storage vessels holding biologically active materials are covered and means for collecting and treating the waste gases connected, or in the case of mixed wastes containing water the material is anaerobically digested in achieving a controlled methanogenesis.
It is often the case that biodegradable materials occur discontinually or are conditioned batchwise in making use of them biologically. For an optimum or steady feed of the downstream bioreactors, buffering the materials is consequently necessary. When subsequently treated aerobically, aerated storage of materials materializes automatically. But if the material is to be digested anaerobically, storage with exclusion of air is obvious, since aerobic storage uses a lot of energy for aerating the materials, resulting in a relevant conversion of potentially methanogenic substances into carbon dioxide.
When the material is anaerobically stored, the anaerobic biodegradable material is subjected to a chain of degrading reactions. Where organic solids are concerned, this chain involves hydrolysis of the solids, acidification of the dissolved intermediate products (acidogenesis), conversion of the resulting acids into acetic acid, hydrogen and carbon dioxide (acetogenesis) ending in the formation of methane (methanogenesis). Responsible for each step in this conversion are certain groups of microorganisms in each case. When this chain of interdependent reactions is balanced, i.e. when the conversion rates of each step in the reaction are equal, the products of a partial step are further made use of in subsequent steps and there is no accumulation of intermediate products, as a result of which the biodegradable organic carbon is converted into methane and carbon dioxide.
However, the various activities involved in the groups of microorganisms may also result in enrichment of intermediate products, it being mostly the case that the spontaneous anaerobic degrading of dissolved biodegradable substances results in enrichment of organic acids in the substrate, since the activity of acidogenic microorganisms is significantly higher than that of methanogenous substances. When the quantity of enriched organic acids exhausts the buffer capacity, the result is a drop in the pH which in turn results in a reduction in the activity of methanogenic microorganisms. The result of this imbalance is an acidified material the low pH of which totally inhibits methanogenesis, a typical example for this stabilizing process being silage from grass cuttings in agriculture.
This self-inhibition of a completely anaerobic biodegradation of biodegradable organic materials comes up against its limits, however, when the biodegradable material in the substrate mix exists mainly particulate and insoluble, with a low potential of readily acidifiable components, a high buffer capacity and when the density of methanogenic microorganisms is elevated. When this is the case, the resulting organic acids are buffered and there is no significant drop in the pH of the substrate mix, resulting in the methanogenic activity being maintained and hydrolysis being the step determining the rate in anaerobic biodegradation (Noike et al. (1985): Characteristics of Carbohydrate Degradation and the Rate-Limiting Step in anaerobic Digestion, Biotechnology and Bioengineering 27, pp 1482-1489).
In actual practice such conditions exist, for example, in anaerobic digestion of biowastes from selective wastes collection. Depending on the time of year involved these wastes feature a relative low percentage of soluble, readily digestable organic matter but a considerable percentage of particulate biomass (e.g. garden waste). Furthermore, wastes of this kind are often mashed with process water before digestion (EP 0 520 172, DE 198 33 776, DE 199 07 908). This process water is preferably obtained from dewatering digested waste, it thus containing both an elevated density of methanogenic microorganisms and a high buffer capacity. The buffer capacity materializes in the digestion of the methane itself from formation of hydrocarbonates, mainly ammonium hydrocarbonate. TAC values of 4 to 8 g/l are often found in the process water. In anaerobic storage of the suspension the result of this is the spontaneous formation of organic acids being too weak to substantially lower the pH and the methanogenic activity from the process water is sufficient to convert organic acids formed as a result of the solids hydrolysis into methane. Methane and carbon dioxide are thus generated in the storage tank from part of the organic carbon.
When methane is formed in storage of the suspension, connecting the bioreactor to a means for collecting biogas is an obvious process solution, as disclosed in EP 1 280 738. The drawback in this aspect is, however, that by connecting the storage vessel to the biogas collection system the fluctuations in the quality of the biogas become even more pronounced. The biogas formed in the storage vessel is characterized by a low content of methane and a high content of carbon dioxide due to the predominant anaerobic reactions in hydroloysis and acidification. Furthermore, the materials supplied to this storage vessel often feature greatly fluctuating volume flows in brief intervals whilst the storage vessel material is tapped relatively consistently, resulting in heavy fluctuations in the levels in the storage vessel.
When material is supplied to the storage vessel, low-methane biogas is displaced from the reactor into the biogas collection system, resulting in addition to the flow of biogas from the digesters a high volume flow of biogas having a low methane content, causing a brief drop of the methane content in the biogas being produced at the time. On completion of the feed to the storage vessel, the level therein drops and a total collapse in the flow of biogas from the storage vessel may occur, resulting in a strong increase in the methane content in the biogas prompting corresponding fluctuations in the calorific value. Since the systems recycling biogas are designed on the basis of the calorific value of the biogas, such fluctuations in the calorific value disrupt operation in making use of the biogas. This can only be avoided by installing a corresponding large biogas storage capacity which, however, adds to the costs of investment and operation. Furthermore, connecting the storage vessel to the biogas collection system results in a drop in the mean methane content in the biogas and thus a deterioration in quality.
For example, digestion of 70 t of biowaste from the separated collection of domestic waste produces a biogas volume flow of approx. 7,200 m3/d daily. When distributed over the full day this biogas production results in a mean volume flow of 300 m3/h and a methane content of approx. 60% by volume. But conditioning the waste material is done batchwise and the resulting waste suspension is discharged with a volume flow of approx. 160 m3/h into the storage vessel. Because of the displacement this results in an additional biogas flow of 160 m3/h with a methane content of approx. 20% by volume. This in turn briefly results in a biogas volume flow of 460 m3/h with a methane content of approx. 46% by volume, in other words, the calorific value of the biogas drops briefly by almost 25%.
DE 198 33 776 shows the necessity of providing a storage vessel upstream of the digestor but with no indication of how to avoid gas emissions from the storage vessel. Although in EP 1 280 738 connecting the storage vessel to the biogas or digestion gas collection system is described, making such a connection results in trouble in operation, in the absence of a sufficient digestion gas storage volume when making use of the digestion gas, due to the fluctuations in the calorific value.